Method for grain refinement of magnesium alloy castings

ABSTRACT

A method for grain refinement of magnesium alloy castings includes adding graphite (C) powder and manganese dioxide (MnO 2 ) to a melt of magnesium alloy containing aluminum (Al) and manganese (Mn) to finely divide crystal grains of a cast structure.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method for grain refinement ofmagnesium alloy castings, which method is capable of grain refinement ofsolidified structure of the cast product and improving the mechanicalproperties thereof without generating dioxin.

[0003] 2. Description of the Prior Art

[0004] The method for the grain refinement of aluminum (Al)-containingmagnesium alloy, such as the AZ type, is known in two kinds, namely amethod that shuns addition of a grain refiner and a method thatnecessitates addition of the grain refiner.

[0005] The former method includes a method of superheat-treatment thatcomprises superheating he whole of molten alloy to about 850° C. to 900°C. (1123K to 1173K), holding it for about 5 minutes to 15 minutes (0.3ks to 0.9 ks), cooling it at a rate of 150° C./min or more, andthereafter quenching it at the casting temperature. The mechanism ofgrain refinement of the superheat-treatment is considered to nucleate anAl—MN—(Fe) compound. Since this method of superheat-treatment requiresan inevitably high treating temperature, however, it incurs undue costof energy, boost the expense for preventing the molten alloy fromoxidation and performing works of maintenance and inspection on thecrucible, and entails the problem of imperiling economy and safety.

[0006] The latter method includes carbon addition that comprises addinga carbon (C)-containing compound at a temperature in the neighborhood of750° C. to the molten alloy. The mechanism of grain refinement by carbonaddition is considered to nucleate aluminum carbide (Al₄C₃) resultingfrom the reaction of carbon in a compound with aluminum in the moltenalloy. Commercially, the practice of adding hexachloroethane (C₂Cl₆) asthe grain refiner has prevailed. Since the hexachloroethane inducesgeneration of dioxins {2,3,7,8-tetrachloro-dibenzo-p-dioxin:Cl₂(C₆H₂)O₂(C₆H₂)Cl₂}, the use thereof is now prohibited.

[0007] The latter method further includes a method of Carbon Addition byBlowing that comprises directly blowing a graphite (C) powder togetherwith argon gas (Ar) into the molten alloy (refer, for example, to JP-A2003-41331). This method of Carbon Addition by Blowing is capable ofgrain refinement of the solidified structure without inducing generationof dioxins.

[0008] The carbon that is added by blowing, however, can promote thegrain refinement of the solidified structure proportionately to the riseof the temperature of the molten alloy. Though the method of CarbonAddition by Blowing indeed defies generation of dioxins, it requiresheightening the temperature of treatment for the purpose of promotingthe grain refinement of the solidified structure and entails the problemof incurring the cost of energy. This invention aims to provide a methodfor fine grain refinement of magnesium alloy castings while encouragingdecrease of the cost of energy, promoting the grain refinement of thesolidified structure and improving the mechanical properties of themagnesium alloy castings without inducing generation of dioxins.

SUMMARY OF THE INVENTION

[0009] For the sake of accomplishing the object described above, themethod contemplated by this invention for fine gain refinement of themagnesium alloy castings comprises adding a graphite (C) powder andmanganese dioxide (MnO₂) into a melt of magnesium alloy containingaluminum (Al) and manganese (Mn), thereby enabling crystal grains of acast structure to be refined finely.

[0010] In the method for fine grain refinement of the magnesium alloycastings, the addition of a graphite (C) powder and manganese dioxide(MnO₂) embraces the case of them being added as contained in a metalliccapsule formed of pure aluminum, aluminum alloys, pure magnesium ormagnesium alloys.

[0011] The amount of the manganese dioxide ((MnO₂) to be added is in therange of 0.10 mass % to 0.22 mass %, preferably in the range of 0.20mass % to 0.22 mass %, based on the mass of the molten magnesium alloymentioned above.

[0012] According to this invention, since a graphite powder andmanganese dioxide are added to the molten magnesium ally, thetemperature of the molten alloy can be elevated by the reaction ofoxidation-reduction with the manganese dioxide. Since both the additionof hexachloroethane and the generation of dioxins are absent, the costof energy can be decreased, the grain refinement of a crystal can bepromoted, and the mechanical properties of the cast article of magnesiumalloy can be improved. Then, since the graphite powder and the manganesedioxide are allowed to be contained in a metallic capsule, the graphitepowder and the manganese dioxide can be prevented from being scatteredand can be utilized efficiently in small amounts for the grainrefinement Further, since the metallic capsule is made of aluminum alloyor magnesium alloy, they can participate in effecting the grainrefinement without adversely affecting the components of the compositionof the alloy. Since manganese dioxide is added in an amount in the rangeof 0.10 mass % to 0.22 mass % based on the mass of the molten magnesiumalloys, it enables the crystal gains to be finely divided to an averageparticle diameter of 170 μm or less. Since manganese dioxide is added inan amount in the range of 0.20 mass % to 0.22 mass % based on the massof the molten magnesium alloys, it enables the grain refinement to beeffected to the same degree as the addition of hexachloroethane.

BRIEF EXPLANATION OF THE DRAWING

[0013]FIG. 1 shows a schematic illustration of an apparatus used in themethod for fine grain refinement according to this invention.

[0014]FIG. 2 shows a press diagram illustrating a procedure of thisinvention, starting with melting alloy and finishing with casting themolten alloy in a metallic mold.

[0015]FIG. 3 is a diagram illustrating changes of the temperature in aphosphorizer and the temperature of the melt caused by the addition ofmanganese dioxide.

[0016]FIG. 4 is a copy of optical microphotogaphs showing the solidifiedstructure of an untreated material and the solidified structure of amanganese dioxide-added treated material resulting from adding manganesedioxide in an amount of 0.20 mass % based on the mass of molten alloy.

[0017]FIG. 5 shows a copy of optical microphotographs showing thesolidified structure of a hexachloroethane-added treated material, thatof an untreated material and that of a capsule-added treated material,respectively.

[0018]FIG. 6 shows a copy of optical microphotographs showing thesolidified structures of graphite-added treated materials having theamount of added graphite sequentially increased

[0019]FIG. 7 shows a characteristic diagram showing the relation betweenthe amount of graphite added and the average particle diameter ofcrystal grains.

[0020]FIG. 8 shows a copy of optical microphotographs showing thesolidified structures of various complex-added treated materials havingthe amount of added graphite varied.

[0021]FIG. 9 shows a characteristic diagram showing the relation betweenvarious complex-added materials and the average particle diameters ofcrystal grains.

[0022]FIG. 10 shows a copy of optical microphotographs showing thesolidified structures of various complex-added treated materials havingthe amount of added MnO₂ varied.

[0023]FIG. 11 shows a characteristic diagram showing the relation bevarious complex-added materials and average particle diameters ofcrystal grains.

[0024]FIG. 12 shows a copy of an optical microphotograph showing thesolidified structure of a complex-added treated material using agraphite powder having a diameter on the order of nm.

[0025]FIG. 13 shows a characteristic diagram showing tensile strength,0.2% proof strength (MPa) and elongation (%) in the F specimen of anuntreated material, a manganese dioxide-added treated material, ahexachloroethane-added treated material and a complex-added treatedmaterial.

[0026]FIG. 14 shows a characteristic diagram showing tensile strength,0.2% proof strength (MPa) and elongation (%) in the T4 specimen of anuntreated material, a manganese dioxide-added treated material, ahexachloroethane-added treated material and a complex-added treatedmaterial.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0027] The method according to this invention for fine grain refinementof the magnesium alloy castings comprises adding a graphite (C) powderand manganese dioxide (MnO₂) to a melt of magnesium alloy containingaluminum (Al) and manganese (Mn) to enable crystals of a cast structureto be finely refined The magnesium alloy containing aluminum andmanganese does not need to be particularly restricted with respect toits composition. The AZ9E alloy that contains aluminum in thecomposition and manganese as an impurity is used also for sand castingas will be described specifically herein below may be used as suchmagnesium alloy.

[0028] The graphite powder to be used in this invention does not need tobe particularly restricted to the graphite 5 μm in particle diametermentioned in an example cited herein below. Commendably, it has thesmallest possible diameter. Good results are obtained by using agraphite powder having a diameter of the order of nanometer (nm), forexample.

[0029] The manganese dioxide to be used in this invention is depicted asa powder 100 μm in particle diameter in an example to be cited hereinbelow, it may be formed massively (such as in the shape of tablets andpellets). The amount (mass %) of the manganese dioxide to be added fallspreferably in the range of 0.10 mass % to 0.22 mass % and morepreferably in the range of 0.20 mass % to 0.22 mass % eased on theamount (mass) of the melt of magnesium alloy. If the amount of manganesedioxide added falls short of 0.10 mass %, the shortage will result inpreventing the effect of grain refinement from being manifested fullysatisfactory. If the amount of manganese dioxide added exceeds 0.22 mass%, the excess will not bring a proportionate addition to the effect ofgrain refinement. Incidentally, the particle diameter level of themagnesium alloy castings in an untreated state is about 30 μm The effectof grain refinement becomes discernible when the particle diameter fallsshort of this level. Since the solidified structure of alloy ispreferred to possess the finest possible particle diameter, theconditions for obtaining as high an effect of grain refinement aspossible are aimed at a particle diameter which equals the particlediameter attained by the method resorting to the addition ofhexachloroethane.

[0030] In this invention, it is commendable to add a graphite powder andmanganese dioxide as contained in a metallic capsule. By having thegraphite powder and manganese dioxide contained in a metallic capsule,it is made possible to prevent the graphite powder and manganese dioxidefrom being scattered and utilize them efficiently in small amounts forthe grain refinement Further, since the metallic capsule is made of purealuminum or aluminum alloy or of pure magnesium or magnesium alloy, theyare enabled to effect the grain refinement without adversely affectingthe composition of the alloy.

[0031]FIG. 1 shows a schematic view illustration of the apparatus thatis used in the examples of this invention. Referring to FIG. 1,reference numeral 1 denotes a crucible that is formed of iron(Fe)-chromium (Cr)-based SUS 430 stainless steel (Fe-18% Cr) containingno nickel (Ni) and is manufactured by bending this stainless steel platein a cylindrical shape and gas welding the seam. Further, for thepurpose of enhancing the ability to resist high-temperature oxidation,the crucible is subjected to immersion plating in a pure aluminum bathand the layer of pure aluminum formed thereon is subjected to thermaldiffusion so as to coat the surface of the crucible 1 with a FeAl₃ layersparingly liable to be wetted by magnesium. All the casting implementsincluding the crucible 1 are coated with magnesium oxide of the reagentchemical grade so as to prevent inclusion of impurities during themelting of alloy.

[0032] Denoted by reference numeral 2 is an electric furnace and bynumeral 3 is a phosphorizer. The phosphorizer 3 which is destined to bepassed through a hole in the lid of the electric furnace 2 and immersedin molten alloy 11 held in the crucible 1 is made to hold therein ametallic capsule 4 accommodating a graphite (C) powder or manganesedioxide (MnO₂), or accommodating a graphite powder and manganesedioxide. Incidentally, the phosphorizer 3 is manufactured in the samemanner as the crucible 1. The metallic capsule is formed of purealuminum or aluminum alloy, or pure magnesium or magnesium alloy. As thegraphite powder, graphite (purity of 99.9and particle size of about 5μm) made by Kojundo Kagaku Kenkyusho Ltd. and sold under the productdesignation of “Pure Carbon.” was used. As the manganese dioxide, areagent chemical grade manganese dioxide (purity of 99.9% and particlesize of about 100 μm) made by Takaraitesuku K. K. was used.

[0033] Denoted by reference numerals 5 and 6 are thermocouples. Thethermocouple 5 is set inside the electric furnace 2 for the purpose ofmeasuring the temperature of the interior of the electric furnace 2, andthe thermocouple 6 is set in the phosphorizer 3 next to the metalliccapsule 4 to measure the temperature of the molten alloy 11. Atemperature-controlling device is labeled with 7. It is intended tocontrol the temperature of the interior of the electric furnace 2,namely the temperature of the molten alloy 11 at a prescribedtemperature, based on the output of the thermocouple 5. A pen recorderis labeled with 8. It serves to record the temperature of the moltenalloy 11 based on the output of the thermocouple 6.

[0034] Now, the method for testing the effect of the addition ofmanganese dioxide on the grain refinement of crystals will be describedbelow.

[0035] For the test, commercially available AZ91E magnesium alloy havinga composition shown in Table 1 below was used. The test was performedthrough the procedure shown in FIG. 2. The amounts, in mass %, ofgraphite powder and/or manganese dioxide added in the individual runs ofthis test were as shown in Table 2 below. TABLE 1 Al Zn Mn Si Cu Ni FeMg 9.01 0.82 0.22 0.01 0.001 0.002 0.0017 Balance

[0036] TABLE 2 Complex-added Variation in amount of Graphite-addedtreated material MnO₂ added treated material Amount of graphite powderAmount of MnO₂ added Amount of added (%), with added (%), with addedamount graphite power amount of MnO₂ fixed of graphite powder fixedadded (mass %) at 0.20 mass % at 0.02 mass % 0.005 0.04 0.30 0.01 0.020.20 0.02 0.01 0.10 0.04 0.005 0   0.06 0.003 — 0.08 0.001 —

[0037] First, the question whether or not the addition of manganesedioxide induces a rise of temperature was studied. For a start, 700 g ofAZ91E magnesium alloy pickled with nitric acid for the purpose ofdepriving the surface thereof of impurities was melted in the electricfurnace 2, and the resultant molten alloy 11 was heated to 800° C. Then,the metallic capsule 4 accommodating therein manganese dioxide in anamount of 0.20 mass % based on the mass of the molten alloy 11 was setin place in the phosphorizer 3, and the phosphorizer 3 was insertedtogether with the thermocouple 6 into the molten alloy 11. Manganesedioxide was added to the molten alloy 11 while the measurement oftemperature was continued meantime. Then, the molten alloy 11 was leftcooling outside the electric furnace 2 to 700° C. and cast in a metallicmold held at room temperature. An untreated sample was also made withoutadding manganese dioxide, with the object of investigating the effect ofaddition of manganese dioxide on a solidified structure.

[0038] The amount of manganese dioxide to be added was decided, with duerespect to the fact that the manganese content in the AZ91E magnesiumalloy conforming to the ASTM standard shown in Table 3 below is in therange of 0.17 mass % to 0.35 mass %, at the maximum such that the totalamount of the manganese content in the commercially available AZ91Emagnesium alloy and the amount of manganese separated as reduced in themolten alloy 11 in consequence of the addition of manganese dioxide maynot surpass the upper limit of the range of the manganese content in theAZ91E magnesium alloy conforming to the ASTM standard. The reason forusing this maximum is that the actual manganese content conforms theASTM standard and shuns exertion of adverse effect on the components ofthe composition of the alloy. TABLE 3 Al Zn Mn Si Cu Ni Fe Impurity Mg8.1˜9.3 0.40˜1.0 0.17˜0.35 0.20 0.015 0.0010 0.005 0.01 Balance

[0039] When manganese dioxide was added as described above, it inducedan oxidation-reduction reaction and consequently varied the temperatureof the interior of the phosphorizer 3 and the temperate of the whole ofthe molten alloy 11, as shown in FIG. 3. It is clear from FIG. 3 thatthe interior of the phosphorizer 3 showed a discernible sign oftemperature elevation, though instantaneously, to the neighborhood ofabout 1370° C. (1643 K). The temperature of the whole of the moltenalloy 11, however, showed virtually no discernible change. Theoxidation-reduction reaction that was caused by the addition ofmanganese dioxide, therefore, was found to be a reaction that elevatedthe temperature inside the phosphorizer 3 or in a narrow regionencompassing the phosphorizer 3.

[0040] The solidified structure, magnified by an optical microscope, ofan untreated material of the molten alloy 11 having nothing addedthereto and the solidified structure, magnified by an opticalmicroscope, of a manganese dioxide-added treated material havingmanganese dioxide added in an amount of 0.20 mass % based on the mass ofthe molten alloy 11 are shown in FIG. 4. From FIG. 4, it may be safelyinferred that manganese dioxide has no effect of grain refinement of thesolidified structure.

[0041] Now, the effect manifested by the sole addition of a graphitepowder on the grain refinement of the solidified structure was studied.First, 700 g of AZ91E magnesium alloy pickled with nitric acid for thepurpose of depriving the surface thereof of impurities was melted in theelectric furnace 2, and the temperature of the molten alloy 11 waselevated to 800° C. by following the procedure shown in FIG. 2. Then thegraphite powder of a gradually increased amount, namely 0.005 mass %,0.01 mass %, 0.02 mass %, 0.04 mass %, 0.06 mass % and 0.08 mass % each,based on the mass of the molten alloy 11 as shown in Table 2 was addedas held in the metallic capsule 4 to the interior of the phosphorizer 3.The sample which bad the graphite powder added thereto and had undergonethe treatment will be referred to as “graphite-added treated material.”For the purpose of comparing and studying the degree of grain refinementof a solidified structure caused by the addition of graphite, a samplehaving hexachloroethane (C₂Cl₆) added thereto (conditions of addition:temperature of addition of 750° C., amount of addition: 1 mass % basedon the mass of the molten alloy 11) (hexachloroethane-added treadmaterial) was also manufactured. Then, for the purpose of studying theeffect of the metallic capsule 4 formed of magnesium alloy on thesolidified structure, a sample having the metallic capsule 4 alone addedthereto (a capsule-added treated material) was manufactured. Themetallic capsule 4 was a product obtained by preparing a blind containermeasuring 19 mm in outside diameter 15 mm in inside diameter, 20 mm inheight and 2 mm in wall thickness and sealing the container with a lid 2mm in wall thickness.

[0042] The solidified structure, magnified by an optical microscope, ofthe hexachloroethane-added treated material, the solidified structure,magnified by an optical microscope, of the untreated material and thesolidified structure, magnified by an optical microscope, of thecapsule-added treated material are shown in FIG. 5. The solidifiedstructure, magnified by an optical microscope, of the graphite-addedtreated material having the graphite powder content gradually increasedas described above is shown in FIG. 6. The relation (characteristic)between the amount of the graphite powder added and the average crystalgrain diameter is shown in FIG. 7. For the purpose of allowing visualobservation of the structure, the sample obtained by performing asolution treatment under the conditions of 673 K and 14.4 ks with theobject of transforming a eutectic crystal generated by quenching into asolid solution was tested for average crystal grain diameter by theslice method using an optical microscope. The average crystal graindiameters reported hereinafter were determined in the same manner.

[0043] From FIG., 5 which shows that the untreated material and thecapsule-added treated material formed equal solidified structures, itmay be safely inferred that the metallic capsule 4 had no effect of finegrain refinement. Hexachloroethane, however, had an effect of fine grainrefinement as clearly noted from FIG. 5. It is clearly noted from FIG. 6and FIG. 7 that the treated materials having the graphite powder addedin the amounts of 0.005 mass %, 0.001 mass % and 0.02 mass % each had anaverage crystal grain diameter of 300 μm equal to that of the untreatedmaterials. From the data, it may be safely inferred that the graphitepowder had no effect of grain refinement When the amount of the graphitepowder added reached 0.04 mass %, the average crystal grain particlebecame 180 μm. Thus, the effect of grain refinement was attained to acertain extant. When the amount of the graphite powder added furtherrose to 0.06 mass % and 0.08 mass %, the average crystal grain diameterbecame substantially constant at about 70 μm. Thus, the effect of grainrefinement in his case equaled that obtained by the addition ofhexachloroethane. The smallest amount of the graphite powder required tobe added for the sale of grain refinement of the solidified structure,therefore, is 0.04 mass %. For the purpose of obtaining the same effectof grain refinement as the hexachloroethane-added treated material, theamount of addition is required to exceed 0.06 mass %.

[0044] Next, the effect brought by the addition of a graphite power andmanganese dioxide on the grain refinement of the solidified structurewas investigated. First, 700 g of AZ91E magnesium alloy pickled withnitric acid for the purpose of depriving the surface thereof ofimpurities was melted by the electric furnace 2 and the temperature ofthe molten alloy 11 was elevated to 800° C. by following the procedureshown in FIG. 2. Then, the manganese dioxide in a fixed amount of 0.20mass % and the graphite powder of a gradually decreased amount, namely0.04 mass %, 0.02 mass %, 0.01 mass %, 0.005 mass %, 0.003 mass % and0.001 mass % each, based on the mass of the molten alloy 11 as shown inTable 2 were added as held in the metallic capsule 4 to the interior ofthe phosphorizer 3. The material resulting from the addition of acomplex additive, namely graphite powder plus manganese dioxide, and thesubsequent treatment will be referred to as a “complex-added treatedmaterial.”

[0045] The solidified structure, magnified by an optical microscope, ofvarious complex-added treated materials mentioned above are shown inFIG. 8 and the relation (characteristic) between the various complexadditives and the average crystal grain diameters is shown in FIG. 9. Itis clearly noted from FIG. 5 and FIG. 9 that the amounts of the graphitepowder added were 0.04 mass % and 0.02 mass %, the average crystal graindiameters became substantially constant at about 70 μm. Thus, the effectof grain refinement of the solidified structure obtained here was equalto that obtained by the addition of hexachloroethane. When the amount ofthe graphite powder added was 0.01 mass % or less, however, the trend ofthe crystal grains toward coarsening proportionately to the decease ofthe amount of addition of the graphite powder became visible. Adiscussion based on FIG. 6 to FIG. 9, therefore, leads to an inferencethat in the case of the addition of the graphite powder in a fixedamount, the complex-added treated material had a higher effect of grainrefinement of the solidified structure than the treated material whichhad no graphite powder added thereto and that the complex-added treatedmaterial having the graphite powder added in an amount of 0.02 mass % ormore obtained the same effect of gain refinement of the solidifiedstructure as the addition of hexachloroethane. This phenomenon may belogically explained by a supposition that in consequence of the additionof both manganese dioxide and the graphite powder, theoxidation-reduction reaction occurred locally at the position actjoining graphite and the elevation of t r e due to the generation ofheat by the reaction promoted the formation of Al₄C₃, a nucleus-formingsubstance. Though the optimum amount of addition of graphite in thecomplex-added treated material is 0.02 mass %, it is commendable for thepurpose of securing the effect of grain refinement of the solidifiedstructure to fix the amount of addition of graphite at 0.01 mass % ormore and 0.1 mass % or less.

[0046] Next, the effect brought by the change in the amount of additionof manganese dioxide on the refinement of crystal grains wasinvestigated. First, 700 g of AZ91E magnesium alloy pickled with nitricacid for the purpose of depriving the surface thereof of impurities wasmelted by the electric furnace 2 and the temperature of the molten alloy11 was elevated to 800° C. by following the procedure shown in FIG. 2.Then, the graphite powder in a fixed amount of 0.20 mass % and themanganese dioxide of a gradually decreased amount, namely 0.30 mass %,0.20 mass %, 0.10 mass % and 0 mass % each, based on the mass of themolten alloy 11 as shown in Table 2 were added as held in the metalliccapsule 4 to the interior of the phosphorizer 3.

[0047] The solidified structures, magnified by an optical microscope, ofthe various complex-added treated materials are shown in FIG. 10, andthe relation (characteristic) between the various complex additives andthe average crystal grain diameters, is shown in FIG. 11. It is clearlynoted from FIG. 10 and FIG. 11 that when manganese dioxide was added inamounts of 0.30 mass % and 0.20 mass %, the average crystal graindiameters became substantially constant at about 70 μm. Thus, the effectof grain refinenent of the solidified structure obtained here equaledthat obtained by the addition of hexachloroethane. When the amount ofaddition of manganese dioxide fell short of 0.10 mass %, however, thetrend of crystal grains toward coarsening proportionately to thedecrease of the amount of addition of manganese dioxide became visible.In the complex-added treated material having the graphite powder addedin a fixed amount, therefore, it is inferred in view of the data of FIG.11 that the addition of manganese dioxide in an amount of 0.10 mass % ormore resulted in bringing the effect of grain refinement having anaverage crystal grain diameter of about 170 μm or less and the additionof manganese dioxide in an amount of 0.20 mass % or more resulted inbinging the same effect of refined crystal grain as in the case ofadding hexachloroethane. Though the optimum amount of addition ofmanganese dioxide in the complex-added treated material is 0.2 mass %,it is commendable for the purpose of securing the effect of grainrefinement of the solidified structure to fix the amount of addition ofmanganese dioxide at 0.10 mass % or more and 0.4 mass % or less.

[0048] Next the effect brought by the particle diameter of graphitepowder on the grain refinement of the solidified structure wasinvestigated. First, 700 g of an AZ91E magnesium alloy pickled withnitric acid for the purpose of depriving the surface thereof ofimpurities was melted by the electric furnace 2 and the temperature ofthe molten alloy 11 was elevated to 800° C. by following the procedureshown in FIG. 2. Then, carbon nanotubes in an amount of the nm order of0.2 mass %, such as 100 nm in diameter and 5 μm in length, based on themass of the molten alloy 11 and manganese dioxide in an amount of 0.20mass % based on the mass of the molten alloy 11 were added as held inthe metallic capsule 4 into the phosplhorizer 3.

[0049] The solidified structure, magnified by an optical microscopes, ofthe complex-added treated material mentioned above is shown in FIG. 12.It is clearly noted from FIG. 12 that the average crystal graindiameters became substantially constant at about 75 μm and the effect ofgrain refinement of the solidified structure obtained here equaled thatobtained by the addition of hexachloroethane. Thus, the graphiteobtained the effect of grain refinement of the solidified structureequaling that obtained by the addition of hexachloroethane even when theparticle diameter of the graphite was changed to the nm order.

[0050] Next, the F specimens (specimens as cast) and the T4 specimens(specimens having undergone a treatment for transformation into a solidsolution, specifically heat-treated at 400° C. for 16 hours) of thevarious treated materials were tested for tensile strength, 0.2% proofstrength (MPa) and elongation (%). First, FIG. 13 is a characteristicdiagram showing the data of tensile strength, 0.2% proof strength (MPa)and elongation (%) obtained of the F specimens of the untreatedmaterial, manganese dioxide-added treated material,hexachloroethane-added treated material and complex-added treatedmaterial, Then, FIG. 14 is a characteristic diagram showing the data oftensile strength, 0.2% proof strength (MPa) and elongation (%) obtainedof the T4 specimens of the untreated material, manganese dioxide-addedtreated material, hexachloroethane-added treated material andcomplex-added treated material.

[0051] It is clearly noted from FIG. 13 and FIG. 14 that the F specimensand the T4 specimens of the complex-added treated materials equaled theF specimens and the T4 specimens of the hexachloroethane-added treatedmaterial in terms of tensile strength, 0.2% proof strength andelongation.

[0052] The preceding example depicted a case of using an AZ91E magnesiumalloy as a magnesium alloy. The castings shown in Table 4 below, forexample, may be used instead so long as they are magnesium alloyscontaining aluminum (Al) and manganese (Mb). As the graphite powder, acarbon nanotube measuring 100 nm in diameter and 5 μm in length has beencited above. Carbon nanotubes measuring 50 nm to 200 nm in diameter and1 μm to 20 μm in length are capable of producing similar effects. TABLE4 Kind of article of Chemical compositions (mass %) cast metal Al Zn MnSi Cu Ni Fe Impurity Mg 1^(st) grade 5.3˜6.7 2.5˜3.5 0.15˜0.35 ≦0.30≦0.25 ≦0.01 — ≦0.30 Balance 2^(nd) grade C 8.1˜9.3 0.40˜1.0  0.13˜0.35≦0.30 ≦0.10 ≦0.01 — ≦0.30 Balance 2^(nd) grade E 8.1˜9.3 0.40˜1.0 0.17˜0.35 ≦0.20 ≦0.015 ≦0.0010  ≦0.005 ≦0.30 Balance 3^(rd) grade 8.0˜10.0 1.5˜2.5 0.10˜0.5  ≦0.3 ≦0.20 ≦0.01 ≦0.05 ≦0.30 Balance 5^(th)grade  9.3˜10.7 ≦0.3 0.10˜0.35 ≦0.30 ≦0.10 ≦0.01 — ≦0.30 Balance ISO5.00˜7.0  2.0˜3.5 0.10˜0.5  ≦0.3 ≦0.2 ≦0.01 ≦0.05 — Balance 1^(st) gradeISO 2^(nd) 7.0˜9.5 0.3˜2.0 ≦0.15 ≦0.5 ≦0.35 ≦0.02 ≦0.05 — Balance gradeA ISO 2^(nd) 7.50˜9.0  0.2˜1.0 0.15˜0.6  ≦0.3 ≦0.2 ≦0.01 ≦0.05 — Balancegrade B ISO 3^(rd)  8.3˜10.3 0.2˜1.0 0.15˜0.6  ≦0.3 ≦0.2 ≦0.01 ≦0.05 —Balance grade

[0053] According to this invention, owing to the addition of manganesedioxide besides a graphite powder to the melt of magnesium alloy, it ismade possible to utilize the reaction of oxidation-reduction caused bymanganese dioxide for elevating the temperature of the molten alloy,promoting the grain refinement of the solidified structure by carbon,and materializing production of a cast article of the magnesium alloywhich enjoys improved mechanical properties. This invention is enabledby the avoidance of addition of hexachloroethane to shun the generationof dioxins and by the utilization of the reaction of oxidation-reductionof manganese dioxide to elevate the temperature of the melt of magnesiumalloy and promote the decrease of the cost of energy consequently andenjoy the same effect as in the case of adding hexachloroethane.

What is claimed is:
 1. A method for grain refinement of magnesium alloycasting, comprising adding graphite (C) powder and manganese dioxide(MnO₂) to a melt of magnesium alloy containing aluminum (Al) andmanganese (Mn) to refine crystal grains of a cast structure.
 2. Themethod according to claim 1, wherein said graphite (C) powder and saidmanganese dioxide (MnO₂) are accommodated in a metallic capsule.
 3. Themethod according to claim 2, wherein said metallic capsule is formed ofpure aluminum or aluminum alloy.
 4. The method according to claim 2,wherein said metallic capsule is formed of pure magnesium or magnesiumalloy.
 5. The method according to claim 1, wherein said manganesedioxide (MnO₂) has an amount in the range of 0.10 mass % to 0.22 mass %based on a mass of the melt of said magnesium alloy.
 6. The methodaccording to claim 2, wherein said manganese dioxide (MnO₂) has anamount in the range of 0.10 mass % to 0.22 mass % based on a mass of themelt of said magnesium alloy.
 7. The method according to claim 3,wherein said manganese dioxide (MnO₂) has an amount in the range of 0.10mass % to 0.22 mass % based on a mass of the melt of said magnesiumalloy.
 8. The method according to claim 4, wherein said manganesedioxide (MnO₂) has an amount in the range of 0.10 mass % to 0.22 mass %based on a mass of the melt of said magnesium alloy.
 9. The methodaccording to claim 1, wherein said manganese dioxide (MnO₂) has anamount in the range of 0.20 mass % to 0.22 mass % based on a mass of themelt of said magnesium alloy.
 10. The method according to claim 2,wherein said manganese dioxide (MnO₂) has an amount in the range of 0.20mass % to 0.22 mass % based on a mass of the melt of said magnesiumalloy.
 11. The method according to claim 3, wherein said manganesedioxide (MnO₂) has an amount in the range of 0.20 mass % to 0.22 mass %based on a mass of the melt of said magnesium alloy.
 12. The methodaccording to claim 4, wherein said manganese dioxide (MnO₂) has anamount in the range of 0.20 mass % to 0.22 mass % based on a mass of themelt of said magnesium alloy.